Question

I have an application, very basic description:

1. Users login -> they post “activities” (like snowboarding)
2. Users have friends
3. Users can bind multiple friends to activities
4. Users can “like” activities created by friends

What I need is a query to check if the user is allowed to “like” an activity by one of their friends. They are only allowed when one of the users friends was bound to the target activity.

users: id, name
usersFriends: id, uid, friendUid
activities: id, description
activitiesUsers: id, activityId, uid
activitiesLikes: id, activityId, uid

I hope someone can help me with this query, and if possible to return true or false. I hope my question is clear and thanks for your time 🙂

Answer 1

This should produce a list of activities.id for all of the user’s friends.

SELECT 
  activities.id AS canLikeId
FROM 
  users u
  JOIN usersFriends uf ON u.id = uf.uid
  JOIN activitiesUsers au ON uf.friendId = au.uid
  JOIN activities a ON a.id = au.activityId

Wrapped in an EXISTS, it looks like:

SELECT activities.id FROM activities aCanLike
WHERE EXISTS (
    SELECT 
      a.id AS canLikeId
    FROM 
      users u
      JOIN usersFriends uf ON u.id = uf.uid
      JOIN activitiesUsers au ON uf.friendId = au.uid
      JOIN activities a ON a.id = au.activityId
    WHERE a.id = aCanLike.id
)

Or something with an IN() clause that attempts to get everything from activities owned by any of the user’s friends.

SELECT
  activities.*
FROM activities JOIN activitiesUsers ON activities.id = activitiesUsers.activityId
WHERE activitiesUsers.uid IN (
  SELECT friendUid FROM usersFriends WHERE uid = $userid
)